JOURNAL OF TELECOMMUNICATIONS, VOLUME 25, ISSUE 1, MAY 2014
1
Design and Realisation of Planar Star Mi-crostrip Diplexer for
Wireless Applications
A. Zakriti1, N. Amar Touhami2, and M. Lamsalli2
AbstractIn this paper, a high performance diplexer is designed
and fabricated for wireless applications. The diplexer mainly
comprises two dual-mode Star Bandpass Filters (BPFs), operated at
1.8GHz and 2.3GHz, respectively. Several transmission zeros are
located at the passband edges, thus improving the passband
selectivity. Due to the impedance matching between two BPFs, a high
isolation greater than 30dB between two channels is obtained.
Index TermsMicrostrip filters, Resonator filters, Passband
filters, Diplexer.
u
1 INTRODUCTIONecently, the need of integrating more than one
com-munication standard into a single system increased the progress
in mobile and wireless communication
technology [1-2]. Consequently, different standards may use
different frequency bands. In these communication systems, the
diplexers are essential components that can be used to connect a
single antenna to several receivers and transmitters or to provide
the coexistence of different wireless sub-systems in multi-service
and multiband communication systems. Diplexers are the key to
achieve high compactness, high weight, and high isolation.
There-fore, there has been much research focused on developing
various kinds of diplexers [3-6].
A diplexer is a three-port network, that usually con-sists of
two filters connected in a special way in order to provide the
passband and stopband characteristics of each filter from the
common connection [7-8]. Microstrip diplexers are low cost devices
that can be easily mounted on the dielectric substrate and can
provide a more flexible design of the circuit layout [9]. Much
effort has been paid to reduce the size and improve the performance
of these diplexers.
Microstrip diplexer/filter based on the common reso-nator
section [10], modified stepped-impedance resona-tors [11,12] and
defected ground structure [13-14] realize a good selectivity, high
isolation and wide stopband. For a diplexer with wide stopband, it
is easy to control the frequency ratio of the two passbands,
because the har-monic of the lower passband is far away from the
higher passbands and will not affect the high passband when the
lower passband moves down.
In this paper, Star-shape microstrip resonators are proposed to
be the main part of the bandpass filters. The designed diplexer is
based on a combination of two pro-posed bandpass filters with the
center frequency at 1.8GHz and 2.3GHz, respectively.
2 DIPLEXER THEORY 2.1 Coupled Resonator Filters The derivation
of the general coupling matrix of a cou-pled resonator filter has
been presented in [15-16]. Electric and magnetic couplings have
been considered separately in the derivation of the coupling
matrix, and a solution has been generalized for both types of
coupling. In the case of magnetically coupled resonators, using
Kirchoffs voltage law, the loop equations are derived from the
equivalent circuit shown in figure 1(a), and represented in
impedance matrix form. Similarly, for electrically coupled
resonators, using Kirchoffs current law, node equations are derived
from the equivalent circuit in figure 1(b), and represented in
admittance matrix form. The derivations show that the normalized
admittance matrix has identical form to the normalized impedance
matrix [15-16]. Ac-cordingly, regardless of the type of coupling, a
general normalized matrix [A] in terms of coupling coefficients and
external quality factors is derived as given in equa-tion (1).
(a) (b)
Fig. 1. (a) Equivalent circuit of magnetically n-coupled
resonators, (b) Equivalent circuit of electrically n-coupled
resonators.
where qei is the scaled external quality factor (qei=Qei.FBW) of
resonator i, mij is the normalized coupling coefficient
(1 ) ENSAT, UAE, Ttouan, Morocco.. (2) Faculty of Sciences, UAE,
Ttouan, Morocco.
R
i1 i2 i3 inR1
R2 R3Rn
L2L1 L3 Ln
C2C1 C3 Cn
M12 M23
M1n Mn1
Mn,n-1M13
esv1
G1 L1C1
is
v2G2L2
C2v3
G3L3C3
vnGnLn
Cn
M12 M23
M1nMn,n-1M13
Mn1
[] =11 0 00 00 0 1
+ 61 0 00 00 0 17 :11 12 1 = (1)
2
(mij=Mij/FBW), FBW is the fractional bandwidth, and the diagonal
entries mii account for asynchronous tuning, so that resonators can
have different self-resonant frequen-cies. P is the complex lowpass
frequency variable given by the following expression: (2)
The transmission and reflection scattering parameters are
expressed in terms of the coupling matrix and the exter-nal quality
factors as follows [15-16],
(3)
(4)
2.2 Coupled Resonator Diplexers The equivalent circuit of a
diplexer consisting of two bandpass filters with T-junction is
shown in figure 2,
Fig. 2. Architecture of diplexer .
The S parameters of a three-port coupled resonator circuit are
expressed in terms of the coupling matrix (Eq. 1) and the external
quality factors as follows:
(5)
(6)
(7)
Where it is assumed that port 1 is connected to resonator 1,
ports 2 and 3 are connected to resonators x and y re-spectively.
The formulae of the remaining scattering parameters S22, S33, and
S32 can be derived analogously to the previous derivations, and
they are given by,
(8)
(9)
(10)
The proposed coupled resonator components may be syn-thesized
using different ways: analytic solution to calcu-late the coupling
coefficients, or full synthesis using EM simulation tools, whereby
the dimensions of the physical
structure are optimized, or optimization techniques to
synthesize the coupling matrix.
3 TREE-POLE STAR BAND-PASS FILTER The designed diplexer is based
on a combination of two Star-shape microstrip bandpass filters. The
two filters are designed independently to achieve the desired
passbands characteristics with the center frequency at 1.8GHz and
2.3GHz, respectively. 3.1 Design of 1.8GHz Band-Pass Filter
Resonator
Filters The basic configuration of the three-pole microstrip
bandpass star filter considered in this paper is illustrated in
figure 3. The bandpass filter consists of three microstrip star
resonators and feeding lines. The coupling between mictrostrip
stars and each of the feed lines is by a cou-pled-line structure.
The proposed filter structure has a 1.58mm thick dielectric
substrate with a relative dielectric constant of 4.3. The overall
response of the BPF is deter-mined by the coupling between the
resonators, thus the coupling coefficient is directly related to
the spacing be-tween the resonators.
Fig. 3. Layout of 1.8 GHz three-pole microstrip Star BPF
(W1=3.1mm, W2=1.2mm, W3=0.4mm, S1=S2=0.2mm, L1=L2=26.4mm). Figure 4
depicts the filters response obtained using simu-lation tools. The
plot shows a 3dB bandwidth of 70 MHz with a center frequency of 1.8
GHz. An insertion loss of less than 0.2dB has been obtained.
Fig. 4. Simulation response of 1.8 GHz three-pole Mi-crostrip
Star BPF. 3.2 Design of 2.3GHz Band-Pass Filter Resonator
Filters
The Star-shape basic structure shown in figure 3 to the feeding
method used for the 1.8GHz filter can be used
= '0 0 + 21 = 2%1. []11 11 = %1 21 []111-
BPF 1
BPF 2
1
2
3
21 = 2%1. []11 31 = 2&1. []11 11 = 1 21 []111
22 = 1 2 []1 33 = 1 2 []1 32 = 2% . []1
W1
W2W3 S1S2
L1
L2
S1,1S2,1
4
When operating at 1.8 GHz more current distribution is located
on the first BPF since the input impedance seen into the second BPF
is infinite, and the path to the second BPF is open. Similarly,
when operating at 2.3 GHz more current distribution is located on
the second BPF since the input impedance seen into the first BPF is
infinite, and the path to the first BPF is open.
(a)
(b) Fig. 8. Simulated current distribution and coupling paths
oscillating of the diplexer at the centre frequency at (a) 1.8 GHz
and (b) 2.3 GHz.
5 CONCLUSION This paper presents the design of a high
performance di-plexer for wireless applications. At the first step,
a new three-pole microstrip Star bandpass filter is designed.
Secondly, two dual-mode BPFs at 1.8GHz and 2.3GHz using Star
filters are used as main blocks to form the pro-posed diplexer.
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Alia ZAKRITI was born in ElHoceima, Morocco. She received the
the Ph.D. degree in Electronic at the Abdelmalek Essaadi
University, Morocco in 2001. During 2004-2010, she is professor
assistant at Caddi Ayad University, Morocco. Since 2010, she joined
the De-partment Of Engineering Technologies:Telecommunications and
Mecatronics (TITM) as an associate Professor of Telecommunica-tions
Engineering, National School of applied Sciences, UAE, Ttouan,
Morocco. Currently his interests are printed microwave passive and
active circuits, Filters and antenna designs. Naima AMAR TOUHAMI,
was born in Tetuan, Morocco. She re-ceived Bachelor in physics,
DESA in Instrumentation and electronics and PhD degrees in
electronics and Telecommunication from Uni-versity Abdelmalek
Essaadi, in 1996, 2002 and 2009 respectively. She received the
AECID scholarship from the Spanish Ministry of Foreign Affairs
(2005-2008) and participated in several research
